Evaluating the performance of the Pain Interference Index and the Short Form McGill Pain Questionnaire among Chilean injured working adults

Background Chronic pain can lead to economic instability, decreased job productivity, and poor mental health. Therefore, reliable identification and quantification of chronic pain is important for clinical diagnosis and treatment. Objective To determine the psychometric properties of the Spanish language versions of the Pain Interference Index (PII) and the Short Form McGill Pain Questionnaire (SF-MPG) among a population of working adults who experienced injury in Santiago, Chile. Methods A total of 1,975 participants with work-related injuries were interviewed to collect sociodemographic, occupational, and chronic pain characteristics. Construct validity and factorial structure of the PII and SF-MPG were assessed through exploratory factor analyses (EFA). Cronbach’s alpha was used to evaluate internal consistency. Results The PII mean score was 3.84 ± 1.43 among all participants. The SF-MPG median score was 11 [IQR: 6–16] in this study population. Cronbach’s alpha for the PII was 0.90 and 0.87 for the SF-MP. EFA resulted in a one factor solution for the PII. A two-factor solution was found for the SF-MPG. The two-factors for SF-MPG were sensory and affective subscales with Cronbach’s alpha of 0.82 and 0.714, respectively. When the two scales were combined, an EFA analysis confirmed the PII and SF-MPG measure different aspects of chronic pain. Conclusions The PII and SF-MPG had good construct validity and reliability for assessing different aspects of chronic pain among working Chilean adults.


Introduction
Despite their increased utility in clinical and population-based studies, to the best of our knowledge, no previous study has validated the PII or SF-MPG in Spanish-speaking populations. Therefore, the objective of our study was to determine the psychometric properties of the (1) Pain Interference Index (PII) and (2) Short Form McGill Pain Questionnaire among a population of working adults who experienced an injury in Santiago, Chile.

Study population
Study participants were from the Stress, Pain, Sleep, and Neuropsychiatric Disorders (SPLEN-DID) Study in Santiago, Chile. The SPLENDID study was conducted between September 2015 and February 2018. The overarching goals of the SPLENDID study were to examine pain, work-related stress, and neuropsychiatric outcomes among working adults attending the Hospital del Trabajador in Santiago, Chile with an intention of designing workplace intervention programs. The Hospital del Trabajador is the biggest workers' compensation hospital and the referral center for trauma and professional diseases of Asociación Chilena de Seguridad, the largest workers' compensation system in Chile, with more than two and a half million affiliated workers. Using convenience sampling, participants were approached and interviewed during their outpatient visit to one of the rehabilitation clinics at the hospital. Participants were eligible for the study if they were able to read and write Spanish and attended the Hospital del Trabajador outpatient departments for the following types of injuries: spinal cord, mild brain injury, bone fractures, burns, and soft tissue injuries of various etiologies. Participants were excluded if they did not read or write Spanish.

Data collection
After informed consent, each participant was interviewed face-to-face by a trained interviewer using a structured questionnaire. The questionnaire ascertained demographic information including age, sex, and education level. Questions were also included regarding participants' occupation and their pain symptoms. The questionnaire was originally written in English, translated into Spanish by a team of native Spanish speakers with experience in pain research. To ensure proper expression and conceptualization of terminologies in local contexts, the translated version was back-translated and modified until the back-translated version was comparable with the original English version. This was important to ensure that the translated questionnaires are conceptually identical to the English versions. All participants provided written informed consent. The institutional review boards of the Hospital del Trabajador, Santiago, Chile, and the Office of Human Research Administration, Harvard T.H. Chan School of Public Health, Boston, MA approved all study procedures and research protocols. Additional information regarding the ethical, cultural, and scientific considerations specific to inclusivity in global research is included in the Supporting Information (S1 Checklist of inclusivity in global research).

Analytic population
A total of 2,000 participants were interviewed for the SPLENDID study. For the purpose of our analysis, 3 participants were excluded for missing information on the PII, 21 participants were excluded for missing information on the SF-MPG and 1 participant was excluded for being outside the study age range . A total of 1,975 participants were included in the final analysis.

Pain Interference Index
The Pain Interference Index (PII) assesses the extent to which pain has interfered with daily activities in the past two weeks prior to assessment [15,16]. PII contains 6 items on a 7-point Likert scale ranging from 0 (not at all) through 6 (complete). Mean PII score ranges from 0-6, with higher scores indicating pain is more likely to interfere with daily activities.

Short-Form McGill Pain Questionnaire
The Short-Form McGill Pain Questionnaire (SF-MPG) is a multidimensional measure of current perceived pain among adults with chronic pain [18]. The main component of the SF-MPG is a 15-item questionnaire comprised of 2 subscales: 1) a sensory subscale with 11 items and 2) an affective subscale with 4 items. Each item is rated on a Likert intensity scale with 0 = none, 1 = mild, 2 = moderate, or 3 = severe. The total SF-MPG score was obtained by summing the item scores (range 0-45). The sensory subscale ranges from 0-33, and the affective subscale ranges from 0-12 [18][19][20].

Covariates
Participants' age was categorized as 18-24, 25-34, 35-44, 45-54, 55-64, 65-74, and 75-84 years. Additional sociodemographic characteristics examined were sex (male vs. female), country of birth (Chile vs. others), belonging to indigenous or native groups (no vs. yes), highest degree of education attained (elementary school, high school, college or technical training), marital status (married/living with a partner, single, or previously married). Height and weight were measured with participants wearing light clothing and no shoes. Body mass index (BMI) was calculated as weight (kg)/height squared (m 2 ). Different thresholds of BMI were set according to the World Health Organization protocol (<18.5, 18.5-24.9, 25-29.9, or >30 kg/ m 2 ). Participants' occupational characteristics examined included work sector (construction, finance, commercial, manufacturing, public services, transportation, or other), type of occupation (administrative, manual worker, professional, salesperson, technician, teacher, or other), time the since accident in days, type of accident (commute vs. work injury), and type of injury (burn, fall, cut, attrition, firearm, blunt trauma, repetitive use, or other).

Statistical analysis
Frequency distributions of sociodemographic and occupational characteristics were assessed using numbers and percentages (%) for categorical variables and mean ± standard deviations (SD) for continuous variables. P-values were calculated using the Chi-square test for categorical variables and the analysis of variance (ANOVA) for continuous variables. For the SF-MPG total score and subscales, median values (± interquartile range [IQR]) were calculated, since the distribution of these values was skewed. The Wilcoxon-Mann Whitney test was used to evaluate differences in medians. Cronbach's alpha was used to assess internal consistency for PII, SF-MPG, SF-MPG sensory subscale, and SF-MPG affective subscale. For both the PII and SF-MPG, we also analyzed each item's reliability by assessing item-total correlations and overall reliability when a specific item was deleted. Construct validity was assessed using exploratory factor analysis (EFA). Prior to conducting the EFA, we examined the data to ensure suitability for EFA; these analyses demonstrated it was appropriate to proceed with factor analyses (For PII: Kaiser's measure of sampling adequacy (MSA) = 0.889, Bartlett's test of sphericity p<0.001; For SF-MPG: MSA = 0.910, Bartlett's test of sphericity p<0.001). EFAs were conducted for the PII and SF-MPG questionnaires both separately and combined. EFAs were conducted using principal component analysis with varimax rotation. Factors with eigenvalues >1 were assumed to be meaningful and retained for rotation. Rotation factor loadings of � 0.4 were considered sufficient while items with factor loadings of � 0.4 on more than one factor were cross-loading. Since previous studies have shown an association between gender and pain tolerance [21,22], exploratory analyses were performed separately for men and women. All statistical analyses were performed using SPSS Statistics, Version 23.0 (IBM SPSS v23.0, Armonk, NY, USA).

Results
Sociodemographic characteristics of the population are presented in Table 1. Participants had a mean age of 45.87 ± 13.69 years. The majority of participants were men (72.4%), between 45-54 years (25.6%), reported high school as the highest degree of education they had completed (54.7%), were married or living with a partner (61.7%) and had a BMI that classifies them as overweight (43.2%). Compared to men, women in our study were more likely to be older, more highly educated, and less likely to be married or living with a partner (p<0.001, Table 1). Characteristics of study participants stratified by age group is also presented (S1 Table). Occupational, injury, and pain characteristics of the population are shown in Table 2. The majority of participants listed their occupation as manual workers (58.0%) and the median time since accident was 189 [IQR: 71-539] days. Nearly two-thirds of participants were injured during work (65.4%). Falls (35.4%) or blunt trauma (32.7%) were the most common causes of injury. The PII mean score was 3.84 ± 1.43 among all participants. The SF-MPG median score was 11 [IQR: [6][7][8][9][10][11][12][13][14][15][16]. Women were significantly more likely to report higher PII and SF-MPG scores than men (p<0.001; Table 2). Overall, those in the older age ranges (35-54 years and 55-88 years) were more likely to have higher values of the PII and SF-MPG scores versus younger participants (18-34 years) (S2 Table).
The Cronbach's alpha reliability coefficient for PII total score was 0.90 and was 0.87 for SF-MPG total score ( Table 3). The SF-MPG sensory and affective subscales had Cronbach's alphas of 0.82 and 0.71 respectively ( Table 3). The item-total correlations coefficients between six items of the PII and the total scores ranged from 0.66 to 0.82 (all p-values <0.001) (S3 Table). The highest item-total correlation coefficient was for item 2 'has your pain made it difficult for you to do activities outside work?' (0.83) and the lowest was for item 5 'has your pain affected your ability to do physical activities?' (0.66; S3 Table). For SF-MPG, the highest item-total correlation coefficient was for item 4 'punishing-cruel pain' of the affective subscale (0.629) while item 10 'tender pain' of the sensory subscale had the lowest correlation (0.312) (S4 Table).
The internal consistency and item-total correlations were similar for the pain scales when examined separately for men and women (S4-S9 Tables).
Exploratory factor analysis showed a one-factor solution for the PII which accounted for 66.93% of the total variance in our population ( Table 4). The results of EFA for SF-MPG among Chilean adults showed a two-factor solution with Factor 1 (Sharp Pain) and Factor Two (Aching Pain). The SF-MPG sensory subscale items 5, 6, 7, 8, 11 and affective subscale items 2, 3, and 4 loaded to Factor 1; while the other items of each subscale loaded to Factor 2. The two factors explained 53.22% of the total variance (Table 5). We performed an EFA analysis that included all 17-items from both pain scales. The results showed a 3-factor solution together explaining 57.76% of total variance. All items from SF-MPG loaded on two factors "Sharp Pain" and "Aching Pain" while the rest of the times from the PII scale loaded on "Pain Interference" (S5 Table).

Discussion
The Spanish-language versions of the PII and SF-MPG had good construct validity, concurrent validity, and internal consistency for assessing chronic pain when administered to injured working Chilean adults. The EFA results indicated that the PII was a unidimensional scale while the SF-MPG had a two-factor solution. Furthermore, the EFA analysis on the pooled items resulted in a 3-factor structure of "Sharp Pain", "Throbbing Pain", and "Pain Interference".
Three previous studies have examined the psychometric properties of PII [15,23,24]. Martin et al examined the PII among 60 English-speaking patients (ages 6 to 24 years) and their parents in Sweden. The PII showed good internal consistency with a Cronbach alpha of 0.84 for the patients, and 0.94 and 0.96 for the parents (mothers and fathers, respectively) [15]. Among children and adolescents at a tertiary pain clinic in Sweden [N = 163, ages 7 to 18 years), Holmström et al found a one-factor solution with a Cronbach's alpha of 0.856 for a Swedish-language version of the PII [23]. In a similar patient population of 205 Swedish adults at a tertiary pain clinic (ages 18 to 85 years), the PII showed a one-factor solution and an overall Cronbach's alpha of 0.85 [24]. Taken together, these results are concordant with the results of our study of the Spanish-language version of the PII used among Chilean adults. Namely, like the previous studies, we found a one-factor solution and a Cronbach's alpha reliability coefficient of 0.90 for the PII. To our knowledge, PII has not been previously validated in Spanish, but other pain scales have been translated to measure their adequacy in Spanishspeaking populations with back injuries, obtaining good assessments of chronic pain [25,26].  [18,19]. The Cronbach's alpha for the final model was 0.77 [19]. Lastly, a study of 100 female Swedish patients (50 with fibromyalgia and 50 with rheumatoid arthritis) showed a three-factor solution with factors of acute sensory, chronic sensory, and affective pain [27]. In our population of working Spanish-speaking Chilean adults, the Table 3 (N = 1,975).  SF-MPG loaded as two factors with both sensory and affective items loading to each of the factors. In other Spanish-speaking populations, Lázaro et al. evaluated the internal consistency of the long form of the MPG, confirming its psychometric validity, but no reliability studies were conducted [28]. As for the SF-MPG, we did not find applications of the Spanish version of this index, which shows the relevance of our findings to identify pain in Spanish-speaking countries. The strengths of our study included large sample size, the use of well-trained interviewers for data collection, and our conduct of sensitivity analyses designed to explore the influence of gender on the chronic pain scales. However, our study also had some limitations. Our population of working adults at a workers' compensation hospital may not be generalizable to other adult populations in South America, because it focuses on injured workers, excluding informal workers, those beneficiaries of the public system, and non-injured workers. Thus, future studies are needed to include these participants and evaluate if the instruments perform similarly. Additionally, we were unable to evaluate criterion validity, since there is no gold standard for measuring the impacts of chronic pain. It is also possible that translation differences of the SF-MPG may have affected the factor structure. Our EFA showed a two-factor solution for the SF-MPG. Previous studies have shown two-factor structures for English and French versions of the SF-MPG [18,19] and a three-factor solution for the Swedish version [27].

Conclusions
Our study was the first to evaluate the reliability, construct validity and concurrent validity of PII and SF-MPG among Spanish speaking Chilean adults. Overall, both the PII and SF-MPG have good construct validity and reliability for assessing different aspects of chronic pain in a Supporting information S1 Table. Sociodemographic characteristics of injured working adults in Chile by age groups (N = 1,975). Table. Occupation, injury, and pain characteristics of injured working adults in Chile by age groups (N = 1,975).